It seems the migration came from a previous incarnation of Economics.se which was closed. A new Economics Beta was started with the same subdomain so the URL happens to be valid—but unrelated.
I'm not sure how I would search for migrated questions from now-closed sites so I'm not sure, but I would guess that in most cases there would be a dead link and in this case it is just pure luck that the URL points to a valid post. I'm also not sure if these is an extreme edge-case or not.
Is this working as expected? And if so is it possible for migration notices to be auto-updated when sites are closed and/or URLs change?